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A commercial head-mounted display (HMD) for virtual reality (VR) presents three-dimensional imagery with a fixed focal distance. The VR HMD with a fixed focus can cause visual discomfort to an observer. In this article, we propose a novel design of a compact VR HMD supporting near-correct focus cues over a wide depth of field (from 18 cm to optical infinity). The proposed HMD consists of a low-resolution binary backlight, a liquid crystal display panel, and focus-tunable lenses. In the proposed system, the backlight locally illuminates the display panel that is floated by the focus-tunable lens at a specific distance. The illumination moment and the focus-tunable lens' focal power are synchronized to generate focal blocks at the desired distances. The distance of each focal block is determined by depth information of three-dimensional imagery to provide near-correct focus cues. We evaluate the focus cue fidelity of the proposed system considering the fill factor and resolution of the backlight. Finally, we verify the display performance with experimental results.

The binder in hot-mix asphalt (HMA) is aged (oxidized) in a short-time period during haul-and-queue in the field. Since the oxidative aging of asphalt is a complex chemical process, it is difficult to define the asphalt aging as a function of limited variables. When comparing the same types of asphalt mixes, however, the mix temperature (T) and time (t) kept at the T will be the primary source of variation affecting aging levels of the binder in the mix. Since the binder aging level is not easy to estimate without measuring a physical property, this study concentrated on developing an aging quantity (AQ) model for estimating aging levels of the binder in the mix based on T and t. The loose asphalt mixes were artificially short-term aged at various Ts for different t; 130, 160, and 180 °C for 1, 2, 4, and 8 h. The absolute viscosity (AV) values, which represent aging levels of the recovered binder after each short-term aging (SA) of normal dense-graded mix, were used for regression with AQ values computed by the AQ model. The best-fit AQ model was selected by trial-and-error regression iterations between measured AV and computed AQ. The AQ was then used to estimate AV (EAV) of the aged asphalts in various normal asphalt mixes. It was found that the AQ was useful for estimating AV of the binder in the SA-treated mix, and the EAV by AQ showed an excellent correlation with the measured AV with R2 > 0.99. Therefore, it was possible to conclude that the AQ could be used to predict aging level of various short-term-aged normal asphalt mixes if the materials sources were limited.

We introduce a novel, to the best of our knowledge, method to increase the bandwidth in holographic displays. Here, multi-angle illumination using multiple laser diodes (LDs) is adopted to expand the limited diffraction angle of the spatial light modulator (SLM). To solve the problem of signal repetitions caused by sharing the same SLM pattern, we use a random binary mask (BM). We demonstrate via simulations and experiments that our method effectively increases the bandwidth with sufficient image quality. Furthermore, the speckle noise, a critical issue of the holographic display that decreases the contrast and is potentially harmful to eyes, is reduced by the advantage of incoherent summation in the reconstruction plane. We believe that this method is a practical approach that can expand the bandwidth of the holographic display by alleviating the bottleneck of hardware limitations.

We propose a hologram generation technique to compensate for spatially varying aberrations of holographic displays through machine learning. The image quality of the holographic display is severely degraded when there exist optical aberrations due to misalignment of optical elements or off-axis projection. One of the main advantages of holographic display is that aberrations can be compensated for without additional optical elements. Conventionally, computer-generated holograms for compensation are synthesized through a point-wise integration method, which requires large computational loads. Here, we propose to replace the integration with a combination of fast-Fourier-transform-based convolutions and forward computation of a deep neural network. The point-wise integration method took approximately 95.14 s to generate a hologram of 1024 × 1024 p i x e l s , while the proposed method took about 0.13 s, which corresponds to × 732 computation speed improvement. Furthermore, the aberration compensation by the proposed method is verified through experiments.

We propose a holographic display that expands the space-bandwidth product to the region of high-order diffraction based on temporal multiplexing technique. The simulation results show the bandwidth expansion by nine times with high image quality.

Optimization of the phase-only computer-generated hologram, based on the alternating direction method of multipliers algorithm is proposed. In this work, the phase randomness of the reconstructed object is preserved, which gives an adequate blur effect.

In this Letter, we introduce a computer-generated hologram (CGH) optimization technique that can control the randomness of the reconstructed phase. The phase randomness significantly affects the eyebox size and depth of field in holographic near-eye displays. Our proposal is to synthesize the CGH through the sum of two terms computed from the target scene with a random phase. We set a weighting pattern for summation as the optimization variable, which enables the CGH to reflect the random phase during optimization. We evaluate the proposed algorithm on single-depth and multi-depth contents, and the performance is validated via simulations and experiments.

We propose a compact tomographic near-eye display by combining a micro–electro–mechanical systems (MEMS) scanning mirror device, focus tunable lens, and a single light-emitting diode source. A holographic optical element was used to elaborately focus the light source into the MEMS scanning mirror while providing further miniaturization. We implemented a drastically downsized multifocal display system that expresses a depth range of 4.8 D with eight focal planes by synchronizing these modules. Furthermore, the planar images are optimized to achieve correct retinal scenes at each accommodation state. The simulated and experimental results verify that the suggested near-eye display system can provide three-dimensional virtual images while showing physical feasibility.

Over the past century, as display evolved, people have demanded more realistic and immersive experiences in theaters. Here, we present a tomographic projector for a volumetric display system that accommodates large audiences while providing a uniform experience. The tomographic projector combines high-speed digital micromirror and three spatial light modulators to refresh projection images at 7200 Hz. With synchronization of the tomographic projector and wearable focus-tunable eyepieces, the presented system can reconstruct 60 focal planes for volumetric representation right in front of audiences. We demonstrate proof of concept of the proposed system by implementing a miniaturized theater environment. Experimentally, we show that this system has wide expressible depth range with focus cues from 25 cm to optical infinity with sufficient tolerance while preserving high resolution and contrast. We also confirm that the proposed system provides uniform experience in a wide range of viewing zone through simulation and experiment. Additionally, the tomographic projector has capability to equalize vergence state that varies in conventional stereoscopic 3D theater according to viewing position as well as interpupillary distance. This study is concluded with thorough discussion about tomographic projectors in terms of challenges and research issues.

Near-eye displays (NEDs) for augmented and virtual reality (AR/VR) are spotlighted because they have a possibility to provide much more immersive experiences never possible before. With the virtue of recent progress in sensors, optics, and computer science, several commercial products are already available, and the consumer market is expanding rapidly. However, there are several challenging issues for AR and VR NEDs to become closer to our lives. Here, we will explore these issues and important topics for AR and VR, and introduce some of the ideas to overcome them: diffractive optical elements (DOEs), retinal projection displays, and 3D display with focus cues. First, unlike VR with a simple optical system, AR that needs to merge an artificial image with an outer scene requires additional optics. The diffractive elements have the merit of being thin and transparent, suitable for the image combiner. Among them, holographic optical elements (HOEs) have great potential as they can record the desired volume grating from the simple lens to the complex wavefront using light interference. Second, in order to wear the NEDs for a long time, it must deal with the visual fatigue as well as the form factor. Retinal projection display can effectively prevent the vergence-accommodation conflict problem even with a simple optical design. In the retinal projection display, the light rays from the display are adjusted to converge into a small point using a lens. It ensures a wide depth range in which the images are clearly visible. Furthermore, it is possible to provide observers with accurate focus cues for the alleviation of visual fatigue via multi-layer displays and holographic displays. Recently, we conceived tomographic NED that can reproduce dense focal planes. We confirm that this system provides quasi-continuous focus cues, semi-original contrast, and considerable depth of field. The experimental results of our prototypes are explained. We also explain the recent activities of using deep learning in holographic NED system.

We introduce an approach to expand the eye-box in a retinal-projection-based near-eye display. The retinal projection display has the advantage of providing clear images in a wide depth range; however, it has difficulty in practical use with a narrow eye-box. Here, we propose a method to enhance the eye-box of the retinal projection display by generating multiple independent viewpoints, maintaining a wide depth of field. The method prevents images projected from multiple viewpoints from overlapping one other in the retina. As a result, our proposed system can provide a continuous image over a wide viewing angle without an eye tracker or image update. We discuss the optical design for the proposed method and verify its feasibility through simulation and experiment.

Augmented reality (AR) and virtual reality (VR) have received a lot of attention and expectation with the experiences never possible before. The recent development of the display technology has resulted in high-resolution images indistinguishable from reality. However, unlike the table-top displays, the head-mounted AR and VR displays have additional factors to be considered such as the form factor of the device, the field of view, and the visual fatigue. In this chapter, some of the key technical issues for optically configuring the display in VR and AR are discussed.

We propose a retinal projection type near-eye display with multiple viewpoints. A new optical design can expand the eyebox, a physically constrained problem in the retinal projection display. The proposed method utilizes a MEMS mirror to generate multiple viewpoints based on time multiplexing method. Since the multiple viewpoints are toward the center of the eyeball, it is possible to provide a continuous image corresponding to the eye rotation without eye tracker. We demonstrate proof-of-concept that provides expanded eyebox through the simulation.

In order to implement 3-D displays with focus cues, several technologies, including multi-layer displays, have been introduced and studied. In multi-layer displays, a volumetric 3-D scene is represented by 2-D layer images via optimization process. Although this methodology has been thoroughly explored and discussed in optical aspect, the optimization method has not been fully analyzed. In this paper, we deal with pupil movement that may prevent efficient synthesis of layer images. We propose a novel optimization method called foveated retinal optimization, which considers the foveated visual acuity of human. Exploiting the characteristic of human vision, our method has tolerance for pupil movement without gaze tracking while maintaining image definition and accurate focus cues. We demonstrate and verify our method in terms of contrast, visual metric, and experimental results. In experiment, we implement a see-through near-eye display that consists of two display modules, a light guide, and a holographic lens. The holographic lens enables us to design a more compact prototype as performing the roles of an image combiner and floating lens, simultaneously. Our system achieves $38^\circ \times 19^\circ $ field of view, continuous focus cues, low aberration, small form factor, and clear see-through property.

Conventional occlusion-capable see-through display systems have many practical limitations such as the form factor, narrow field of view, screen-door effect, and diffraction of a real scene. In this Letter, we propose an occlusion-capable see-through display using lens arrays and a photochromic plate. By imaging the occlusion mask on the photochromic plate with near-UV light, the visible light transmittance of the plate changes. Since no black matrix lies on the photochromic plate, our system provides a clear real scene view without the grid structure of the pixels and can prevent diffraction defects of the real scene. We also alleviate the drawback of a narrow field of view using the lens arrays for a reduced form factor.

We introduce a projection-type light field display featuring effective light modulation. By combining a tomographic display with integral imaging (InIm) technology, a novel optical design is capable of an autostereoscopic light field projector. Here, the tomographic approach generates a high-resolution volumetric scene, and InIm makes it possible for the volumetric scene to be reconstructed on a large screen through a projection. Since all the processes are realized optically without digital processing, our system can overcome the performance limitations associated with the number of pixels in the conventional InIm displays. We built a prototype display and demonstrated that our optical design has the potential of massive resolution with a full-parallax in a single device.

The ultimate 3D displays should provide both psychological and physiological cues for depth recognition. However, it has been challenging to satisfy the essential features without making sacrifices in the resolution, frame rate, and eye box. Here, we present a tomographic near-eye display that supports a wide depth of field, quasi-continuous accommodation, omni-directional motion parallax, preserved resolution, full frame, and moderate field of view within a sufficient eye box. The tomographic display consists of focus-tunable optics, a display panel, and a fast spatially adjustable backlight. The synchronization of the focus-tunable optics and the backlight enables the display panel to express the depth information. We implement a benchtop prototype near-eye display, which is the most promising application of tomographic displays. We conclude with a detailed analysis and thorough discussion of the display's optimal volumetric reconstruction. of tomographic displays., Creating realistic 3D displays that are comfortable to use is both a complicated and important problem. Here the authors present a tomographic near-eye display that uses high-speed synchronization of a lens focus and backlight to produce realistic depth cues for effective and comfortable 3D viewing.

Computational accommodation-invariant (AI) display attempts to mitigate vergence-accommodation conflict (VAC) by showing a constant imagery no matter where the observer focuses on. However, due to the usage of an electrically focus-tunable lens, the contrast of imagery is degraded as point-spread functions of multiple foci are integrated. In this paper, we introduce the content-adaptive approach to improve the contrast at the depth of highly salient region in the image. The position of focal plane is dynamically determined considering the zone of comfort and the mean focal distance of salient region. The contrast enhancement compared to conventional accommodation-invariant display is shown through simulation results using USAF resolution target image. We demonstrate our proof-of-concept prototype and its optical feasibility is verified with experimental results.

Currently, commercial head-mounted displays suffer from limited accommodative states, which lead to vergenceaccommodation conflict. In this work, we newly design the architecture of head-mounted display supporting 15 focal planes over wide depth of field (20cm-optical infinity) in real time to alleviate vergence-accommodation conflict. Our system employs a low-resolution vertical scanning backlight, a display panel (e.g. liquid crystal panel), and focus-tunable lens. We demonstrate the compact prototype and verify its performance through experimental results.

Although there have been a desire to implement ideal three-dimensional (3D) displays, it is still challenging to satisfy commercial demands in resolution, depth of field, form factor, eye-box, field of view, and frame rate. Here, we propose shape scanning displays that may have extremely large depth of field (10cm-infinity) without loss of frame rate or resolution, and enough eye-box (7.5mm) with moderate field of view (30°). Furthermore, our prototype provides quasi-continuous focus cues as well as motion parallax by reconstruction of 120 tomographic layers. Shape scanning displays consist of a tunable lens, a display panel, and a spatially adjustable backlight. The synchronization of the tunable lens and spatially adjustable backlight could provide additional dimension of depth information. In summary, we introduce a novel 3D display technology called shape scanning displays that present superior performance in resolution, depth of field, and focus cue reproduction. This approach has a lot of potential to be applied for various field in 3D displays including head-up displays, tabletop displays, as well as head-mounted displays. It could be efficient solution for vergence-accommodation conflict as providing accurate focus cues.

There have been many recent developments in 3D display technology to provide correct accommodation cues over an extended focus range. To this end, those displays rely on scene decomposition algorithms to reproduce accurate occlusion boundaries as well asretinal defocus blur. Recently, tomographic displays have been proposed with improved trade-offs of focus range, spatial resolution, and exit-pupil. The advantage of the system partly stems from a high-speed backlight modulation system based on a digital micromirror device, which only supports 1-bit images. However, its inherent binary constraint hinders achieving the optimal scene decomposition, thus leaving boundary artifacts. In this work, we present a technique for synthesizing optimal imagery of general 3D scenes with occlusion on tomographic displays. Requiring no prior knowledge of the scene geometry, our technique addresses the blending issue via non-convex optimization, inspired by recent studies in discrete tomography. Also, we present a general framework for this rendering algorithm and demonstrate the utility of the technique for volumetric display systems with binary representation.

Hybrid multi-layer displays are proposed as the system combines additive light field (LF) displays and multiplicative LF displays. The system is implemented by integrating the multiplicative LF displays with a half mirror to expand the overall depth of field. The hybrid displays are advantageous in that the form factor is competitive with existing additive LF displays with 2 layers implemented by a half mirror and two panels, only half of brightness loss is experienced compared to multiplicative LF displays with 2 layers, and no time-division is required to provide images for multi-layer displays. The images for presentation planes are processed by light field factorization and optimized with the presented algorithm. Retinal images are reconstructed based on various accommodation states and display types to check the accommodation response and utilized to compare the proposed displays with existing displays. With ray tracing method, retinal images generated by the proposed displays can be obtained. To verify the feasibility of the system, a prototype of hybrid multi-layer displays was implemented and display photographs were captured with different accommodation states of camera. With the simulation results and experimental results, this system was confirmed to support accommodation cues in a range of 1.8 diopters.

We report the effect of thin-film transistors (TFTs) without an active pattern (TFTs w/o an active pattern) on the improvement of the drain current and on the reduction of the drain-induced barrier lowering (DIBL) effect. The electrical characteristics of TFTs w/o an active pattern are immensely different from those of normal TFTs because they are composed of a channel area with wholly preserved excimer laser annealing processed poly-silicon (poly-Si) and a specially designed gate electrode. An un-etched poly-Si layer can be utilized as a channel region by designing the gate metal. The extended electric field following the shape of the gate electrode increases the drain current by six to seven times compared to that of normal TFTs. Furthermore, the extended channel area can reduce the DIBL effect considerably. Since the un-etched poly-Si layer has no sharp slope angles or tips on the edge of the precursor, TFTs w/o an active pattern are highly resistant to breakdowns of the gate insulator. Finally, the reducing photo-lithography process on the precursor layers of TFTs effectively simplifies back-plane fabrication.